1. Field of the Invention
The present invention generally relates to an apparatus for detecting a rotation of a rotational member, and more particularly to an electromagnetic induction-type rotation detector which has a coil of wire disposed around a core with a permanent magnet connected thereto and which is arranged such that the top end surface of the core may face a rotational plane of the rotational member.
2. Description of the Related Art
An electromagnetic induction-type rotation detector has been known, as disclosed in Japanese Utility Model Laid-open Publication No. 56-105847, for example. In this publication, in view of the relationship between a magnetic substance and a rotating member, it is proposed that the magnetic substance is formed in a truncated conical shape so as to optimize a flux density over the whole length of the magnetic substance. That is, the rotation detector in the publication is provided with the magnetic substance formed in the truncated conical shape and a cylindrical bobbin which receives therein the magnetic substance and around which a wire is wound to form a coil.
In Japanese Utility Model Laid-open Publication No. 63-152514, in order to solve such a problem that a space for disposing a permanent magnet is restricted by connecting legs for connecting terminal plates to a coil, proposed is a rotation detector which has a permanent magnet disposed within a housing with an axis of the permanent magnet shifted against that of a core of a magnetic substance. It is described in the publication that the permanent magnet can be enlarged toward a portion having no connecting legs.
Although the space for disposing the connecting legs can be obtained by shifting the axis of the permanent magnet against that of the core as described above, the cross section of the housing has to be enlarged by a portion where the permanent magnet has been shifted. As a result, the overall size of the apparatus will become large. However, this kind of rotation detector including the housing (case) shall be formed as small as possible to be disposed in a limited space. If the axis of the permanent magnet is shifted against that of the core, the permanent magnet will have to be made large in size in order to obtain a predetermined flux density, which may also cause the overall size of the apparatus to become large.